1. Field of the Invention
This invention relates to rotating devices that require some axial and radial end play for a rotor shaft.
2. Prior Art
There are various known ways of mounting a shaft in an electric motor to provide some axial end play for the shaft. This is done because all rotating devices, such as the armature assembly of a wiper motor, require some shaft end play. Various ways for supporting the end of the shaft include a threaded thrust plug with locking nut, but this is typically relatively time consuming to assemble and is unreliable in operation. Another way is to form an injection molded plastic plug so that clearance is obtained by material shrinkage. However, this does not guarantee a 100% clearance due to shaft run out. Also known are various types of retaining metal clips. These have such problems as stacking of tolerances and relatively complex and expensive assembly components.
It would be desirable to have a self-adjusting thrust plug with a low friction thrust bearing surface which minimizes shaft loss during shaft rotation under high thrust and radial loads. These are some of the problems this invention solves by providing a self-adjusting thrust plug.
U.S. Pat. No. 4,321,748 teaches a thermoplastic material used as an end thrust plug and injection molded into the gear housing and when the thermoplastic material cools it automatically provides a clearance between the end of the armature shaft and the concave surface of the molded plug. The amount of clearance will depend on the resin used. Each resin has its own shrinkage rate which determines the amount of clearance. Using both an injection molding press for the first application and a heating device for the second application add cost to the manufacturing process.
The disadvantage of this concept is that if the shaft is not reasonably straight the shaft will orbit at the end that is in contact with the molded concave bearing surface which in turn will increase frictional drag between the mating surfaces. The frictional drag will in turn reduce the motor efficiency during extremely high wiper motor operating loads such as one would experience when the windshield glass is partially dry. The coefficient of friction between the glass and rubber blades is at maximum during this period.
Alternatively, the end thrust plug may be molded and then placed in the cavity of the gear housing where the legs of the thrust plug act as a spring member after it has been stress relieved To stress relieve the legs of the thrust plug requires a secondary heating process.
U.S. Pat. No. 4,199,861 teaches using ultrasonic equipment to sonic weld a thermoplastic collar or bushing to the rotor assembly to a predetermined dimension from one end of the main frame. The horn of the sonic welding equipment is designed in a manner such that it uses the main frame of the electric motor as a stop. The thrust collar or bushing will always be sonic welded to a given dimension. After the sonic welding operation the end plate of the motor is assembled to the main frame where a designed clearance will be present.
The end plate assembly relative to the bearing must be controlled within the designed tolerances in order to be able to obtain the desired end play of 0.003 to 0.018 inches. This is the only dimension that needs to be controlled as an assembly.
U.S. Pat. No. 4,455,498 teaches using the frame and frame end cap to set a fixed axial shaft clearance between the front and rear shaft bearings of the motor assembly by pressing a shaft retainer to a fixed position and using an arbor press with a fixed dimension. Regardless of axial stack-up tolerances of the motor components the axial bearing clearance will always be the same or held very tight.
It would be desirable to have automatic retention and adjustment of an armature assembly of a wiper motor without the aforementioned drawbacks. Additionally, it would be desirable to provide some radial end play for the shaft so as to reduce wear due to any radial misalignment.